JS: Comply with CSP no-unsafe-eval.

[marnixbouhuis]

The current implementation uses Function('return this')() to get the global object. This does not work if you are using protobuf on the web and you disallow unsafe-eval with a CSP header.

This PR changes the way the global object is retrieved: it calls a function and sets this to the global object by calling it with .call(null). Then this is returned.
See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call#parameters

Note: this won't work when you are using js strict mode (use strict).
If running in strict mode, it will use different fallbacks (that work with CSP settings that disallow eval), if none work it will fall back to the current way of getting the global object.

This fixes:

• unsafe-eval in generated javascript code #5464
• google-protobuf struct_pb.js use unsafe-eval conflict with CSP #6770
• js: protocols bundled with package are incompatible with commonjs_strict protobuf-javascript#25 (partially, only the CSP part)

[acozzette]

Thanks, @marnixbouhuis.

[marnixbouhuis]

Thanks for merging!